
COMBUSTION AND FLAME
43
We find that a combustible substance
cannot catch fire or burn as long as its
temperature is lower than its ignition
temperature. Have you ever seen
cooking oil catching fire when a frying
pan is kept for long on a burning stove?
Kerosene oil and wood do not catch fire
on their own at room temperature. But,
if kerosene oil is heated a little, it will
catch fire. But if wood is heated a little,
it would still not catch fire. Does it mean
that ignition temperature of kerosene oil
is lower than that of wood? Does it mean
that we need to take special care in
storing kerosene oil? The following
activity shows that it is essential for a
substance to reach ignition temperature
to burn.
Activity 4.4
(Caution : Be careful while handling
burning candle).
Make two paper cups by folding a
sheet of paper. Pour about 50 mL of
water in one of the cups. Heat both
the cups separately with a candle
(Fig. 4.5). What do you observe?
The history of the matchstick is very
old. More than five thousand years
ago small pieces of pinewood dipped
in sulphur were used as matches in
ancient Egypt. The modern safety
match was developed only about two
hundred years ago.
A mixture of antimony trisulphide,
potassium chlorate and white
phosphorus with some glue and
starch was applied on the head of a
match made of suitable wood. When
struck against a rough surface, white
phosphorus got ignited due to the heat
of friction. This started the
combustion of the match. However,
white phosphorus proved to be
dangerous both for the workers
involved in the manufacturing of
matches and for the users.
These days the head of the safety
match contains only antimony
trisulphide and potassium chlorate.
The rubbing surface has powdered
glass and a little red phosphorus
(which is much less dangerous).
When the match is struck against
the rubbing surface, some red
phosphorus gets converted into white
phosphorus. This immediately reacts
with potassium chlorate in the
matchstick head to produce enough
heat to ignite antimony trisulphide
and start the combustion.
Do these experiences tell you that
different substances catch fire at
different temperatures?
The lowest temperature at which a
substance catches fire is called its
ignition temperature.
Can you tell now why a matchstick
does not catch fire on its own at room
temperature? Why does the matchstick
start burning on rubbing it on the side
of the matchbox?
Fig. 4.5 : Heating water in a paper cup
water
paper
cups
What happens to the empty paper
cup? What happens to the paper cup
with water? Does water in this cup
become hot?